1. Field of the Invention
The present invention relates to lithographic projection apparatus and more particularly to position measurement systems employed therein.
2. Description of the Related Art
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
The term “patterning device” as employed herein should be broadly interpreted as referring to a mechanism that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such a patterning device include:
mask: the concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;                programmable mirror array: an example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        
programmable LCD array: an example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.
In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus—commonly referred to as a wafer stepper—each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Because, typically, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, the pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer.
If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus maybe of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithography apparatus, it is essential to keep an accurate track of the positions of the stages, e.g. a mask stage and a wafer stage, or other components, often in six degrees of freedom. To this end, interferometers are often used. In an interferometer used for position measurement, a measurement beam is reflected by a mirror attached to the target object and brought to interfere with a reference beam that travels an optical path of fixed length. Movements of the target object change the optical path length traversed by the measurement beam and so cause shifts in the interference fringes formed between it and the reference beam. The movements of the interference fringes are counted and used to calculate the movements of the target object. In a heterodyne interferometer, the Doppler shift of the fringes is measured. Highly accurate measurement of displacements of the target object can be achieved.
However, any disturbance of the atmosphere through which the measurement and reference beams pass that causes a change in the optical path length will introduce errors into the measured displacements. Such disturbances may be caused by, for example, changes in the air temperature or pressure, or by leaks of gases, e.g. gases such as He or N used to flush the projection beam path in apparatus using radiation that is strongly absorbed by air.
To maintain the accuracy of position measurements by interferometers, efforts have been taken to provide air showers that supply artificial air of known and constant composition, temperature, and pressure to the measurement and reference beam paths as well providing mechanical devices to reduce turbulence. Such arrangements have sufficed for interferometric displacement measuring systems capable of measuring displacements with an accuracy of the order of 10 nm. However, given the ever present demands to image ever smaller features with higher precision, it is desirable to be able to measure stage positions with even smaller error margins.
Because of the well-characterized dispersion of air, see for example M. E. Thomas and D. D. Duncan, “Atmospheric Transmission,” in F. G. Smith, ed., The Infrared and Electro-Optical Systems Handbook, Volume 2 (U. Michigan Press, Ann Arbor, Mich.) 88, incorporated herein by reference, measurement of an optical path at two sufficiently separated wavelengths can allow the effects of pressure and temperature changes to be determined. The displacements measured by an interferometric displacement measuring system can then be corrected accordingly. A second harmonic interferometer to measure such effects has been proposed in F. A. Hopf, A. Tomita, and G. Al-Jumaily, “Second-harmonic interferometers,” Opt. Lett. 5 (1980) 386. Interferometric displacement measuring devices making use of second harmonic interferometers have been described in U.S. Pat. Nos. 4,948,254, 5,404,222, 5,537,209, 5,543,914 and 5,991,033. However, practical difficulties in the implementation of a two color interferometer in a lithographic projection apparatus remain.